I. Field of the Invention
The present disclosure relates generally but not exclusively to novel methods of using homogenous rhodium catalysts comprising N-heterocyclic carbene ligands for the hydroformylation of olefins and substituted olefins, including for example, the use of (acac)(CO)Rh-Imes as a catalyst for the selective hydroformylation of allyl alcohol to 4-hydroxybutyraldehyde (HBA). Further aspects and specific embodiments of the disclosure are provided below.
II. Description of Related Art
Hydroformylation is a significant and commercially important process in which an alkene is reacted with carbon monoxide and hydrogen to form an aldehyde. (Leeuwen and Claver, 2000). This transformation is an industrially important process, which is used to produce compounds such as 4-hydroxybutyraldehyde (HBA), which is in turn used in the synthesis of 1,4-butanediol. See, for example, U.S. Pat. Nos. 4,065,145, 4,215,077, 4,238,419, 4,567,305, 4,678,857, 5,290,743, and 7,790,932. Rhodium-based complex with phosphine ligands are commonly-used catalysts for hydroformylations. Phosphine ligands have been shown to affect the selectivity as well as the reactivity of the metal catalyst depending on the structure of the ligand (Evans, et al., 1968a; Evans, et al., 1968b; U.S. Pat. No. 3,239,569; U.S. Pat. No. 3,239,570; Slaugh and Mullineaux, 1968; Yagupsky, et al., 1969; Brown and Wilkinson, 1969; Brown and Wilkinson, 1970).
While hydroformylation was discovered decades ago, many challenges remain, including minimizing the formation of undesired co-products and byproducts (Coloquhuon, et al., 1991), preventing catalyst degradation, addressing the sensitivity of the phosphine ligands that are typically used to oxidation (Pruett, et al., 1979), and identifying reaction conditions that do not require the presence of a large excess of phosphine ligand (Brown and Wilkinson, 1969; Brown and Wilkinson, 1970; Hjortkjaer, 1979). Reducing the byproducts and co-products also remains a challenge in the production of the HBA. One such co-product is 3-hydroxy-2-methylpropionaldehyde (HMPA), which is a branched isomer of HBA. While not all co-products are necessarily undesirable, the application of additional energy- and/or capital-intensive steps is typically required to separate them for the main product. The generation “C3-byproducts” such as n-propanol and propionaldehyde also continues to remain a challenge. All side products, regardless of whether they are byproducts or co-products, are produced at the expense of the main product, thereby impacting the overall reaction yield.
In order to improve the production 1,4-butanediol, numerous studies have explored the desired properties of the ligand in order to raise the yield of the desired hydroformylation product. Many of the efforts have been directed towards identifying the optimal ligand type, concentration, and substitution pattern. For example, U.S. Pat. No. 6,127,584 reports the use of a trialkyl phosphine ligand containing at least two methyl groups as one method of improving the ratio of HBA to HMPA. Furthermore, diphosphine ligands such as DIOP, XANTPHOS, or trans-1,2-bis(diphenylphosphinomethyl)cyclobutane have been explored and shown to be effective in improving the HBA:HMPA ratio in studies discussed in Japan Kokai Nos. 06-279345 and 06-279344 as well as in the U.S. Pat. No. 4,306,087. Moreover, studies have been carried out using complex butane and cyclobutane ligands as disclosed in U.S. Pat. Nos. 7,271,295 and 7,279,606. Other studies have explored other components of the reaction such as the concentration and pressure of the carbon monoxide and its effect on the overall production of specific products. The concentration of CO, for example, was identified by U.S. Pat. No. 6,225,509 to play a significant role in the production of byproducts and co-products.
In spite of the advances that have been made, the development of new catalytic hydroformylation processes remains desirable. Depending on the production requirements and desired product specifications, different process parameters and characteristics will be of greater/lesser importance. Such parameters and characteristic include improved product selectivity, energy efficiency, catalytic activity, catalyst turn over number, and catalyst life. The reduction and/or elimination of waste products, atom economy, and improved product yield and purity also remain important considerations. By providing novel hydroformylation processes that offer different product and reaction profiles, additional flexibility is provided for addressing one or more of the challenges faced in the production of hydroformylation products, including HBA.